A planar-transmission-line-to-waveguide adapter such as a stripline-to-waveguide adapter or a microstrip-to-waveguide adapter is an apparatus that implements a transition between a planar transmission line (a microstrip and a stripline are both planar transmission lines) and a circuit waveguide having a three-dimensional structure. The circuit waveguide having a three-dimensional structure is generally a standard waveguide, and a waveguide cavity is padded with no medium.
Bandwidth supported by a standard waveguide is generally wider, while bandwidth supported by a planar-transmission-line-to-waveguide adapter is generally narrower. Therefore, after the planar-transmission-line-to-waveguide adapter and the standard waveguide are used in a coordinative way, the planar-transmission-line-to-waveguide adapter becomes a bottleneck for bandwidth expansion.